1. Field of the Invention
The present invention relates to an organic thin-film electroluminescence (=EL) device for use particularly in a planar light source, a display or the like.
2. Description of the Prior Art
An EL device fabricated by an organic substance as material has been expected to realize a low-cost large-surface multicolored display device. For example, a condensed multi-ring aromatic compound such as anthracene, perylene or the like is made into a thin film an a Langmuir-Blodgett (LB) method, a vacuum evaporation plating method or the like to produce a DC driving organic thin-film EL device and study its luminescent properties. Such prior art organic thin-film EL devices, however, require high driving voltage, whereas their luminous brightness and luminous efficiency are inferior to those of inorganic thin-film EL devices. Further, it shows considerable degradation of luminescent properties and, thus is far remote from practical level.
Recently, however, a new type of an organic thin-film EL device having a double-layer structure of organic thin films was reported to attract keen attention [Applied Physics Letters, Vol. 51, No. 12, pages 913-915 (1987)]. According to the report, this organic thin-film EL device has a double-layer structure as shown in the attached FIG. 2 comprising an organic luminescent thin-film layer 24 fabricated by a strongly fluorescent metal chelate complex and an organic hole injection layer 23 fabricated by an amine series compound, which layers are sandwiched between an indium-tin-oxide (ITO) transparent electrode 22 and a backplate 25, and owing to such double-layer structure enables the device to obtain bright green luminescence. In effect, brightness of some hundreds cd/m.sup.2 reportedly was obtained at DC driving voltage of 6-7 V, and the maximum brightness 1,000 cd/m.sup.2 at 10 V. Further, it shows performance of the maximum luminous efficiency 1.5 lm/W, which is near to practical level.
This device however is of a type driven by a DC current and so, in order to gain such high brightness, requires stable flow of DC current larger than 100 mA/cm.sup.2. The organic luminescent layer of the material used heretofore is not satisfactory to realize such stable flow of a large current, owing to degradation of the luminescent layer by electric conduction and morphological change in the luminescent thin-film which cause reduction of electron and hole injection into the organic luminescent layer.